With just over 24 hours to go until the fiscal cliff deadline, Senate negotiators are still scrambling to come up with a last-minute deal to prevent the huge tax hikes and budget cuts scheduled to go into effect on January 1.

Lawmakers have spent the weekend trying to come up with a deal that can pass both the Senate and the House, but negotiators are reportedly still deadlocked over key provisions, including the estate tax, the extension of unemployment benefits, and cuts in Medicare payments to doctors.

The main issue, however, continues to be disagreement over what the threshold should be for letting the Bush tax rates expire. Democrats are still insisting that tax rates go up for those households earning more than $250,000 a year. Republicans want the threshold to be higher, perhaps around $400,000.

According to the Washington Post, Senate Majority Leader Harry Reid and his Republican counterpart, Minority Leader Mitch McConnell, have set the deadline for negotiations at 3 p.m. Sunday.

If all goes well, the leaders will introduce the legislation Sunday evening and hold a vote by tomorrow afternoon. The House would then have about 12 hours to take action.

If the Senate can't reach a deal, Reid has promised to hold an up-down vote on a scaled-down bill, which would extend unemployment benefits and tax breaks only for those earning under $250,000.

Whatever happens, the next 24 hours will definitely be packed with fiscal cliff action. We'll be updating below as things develop.

John Boehner: "Americans elected President Obama to lead, not cast blame."

House Speaker John Boehner responded Sunday to Barack Obama's interview with Meet The Press, in which the President accused Republicans of obstructing a deal in order to keep taxes low for the wealthy.

In a statement, Boehner called Obama's remark "ironic" and accused the President of failing to lead in fiscal cliff negotiations.

Here's the full statement:

“Americans elected President Obama to lead, not cast blame. The president’s comments today are ironic, as a recurring theme of our negotiations was his unwillingness to agree to anything that would require him to stand up to his own party. Needed cuts and reforms that the president agreed to just last year were no longer on the table, as he cited an inability to sell them to Democrats.

“In an effort to get the president to agree to cut spending -- which is the problem -- I put revenues on the table last year, and I put them on the table again last month. Republicans made every effort to reach the ‘balanced’ deficit agreement that the president promised the American people, while the president has continued to insist on a package skewed dramatically in favor of higher taxes that would destroy jobs. We’ve been reasonable and responsible. The president is the one who has never been able to get to ‘yes.’

“The House has passed legislation to avert the entire fiscal cliff, and the president has never called for the Senate to act on those bills in any way. He instead has simply allowed the Democratic-controlled Senate to sit on them and lead our economy to the edge of the fiscal cliff. I am pleased Senators from both parties are currently working to find a bipartisan solution that can finally pass that chamber. That is the type of leadership America needs, not what they saw from the president this morning.”

“While the President was taping those discordant remarks yesterday, Sen. McConnell was in the office working to bring Republicans and Democrats together on a solution,” spokesman Don Stewart told the Washington Post. “Discussions continue today.”

Republicans have added changes to Social Security spending to their list of fiscal cliff demands, in a last-minute surprise that Democrats are characterizing as a major setback in negotiations.

According to new reports, Republican Senate negotiators are now demanding that any fiscal cliff deal include a switch to the chained CPI in calculating Social Security cost-of-living increases.

During his negotiations with House Speaker John Boehner, President Barack Obama indicated that he would be open to the chained CPI proposal, and several Senate Democrats subsequently said they would be able to stomach its inclusion in a deal. But in the wake of Boehner's decision to walk away from the negotiations, Democrats are unlikely to give in without a concession on the debt ceiling.

A theory on the chained CPI inclusion

National Review editor Robert Costa suggests that Republican Minority Leader Mitch McConnell's new insistence on the chained CPI might be a political tactic aimed at helping House Speaker John Boehner save face by boosting support for a deal among GOP House members.

Reid Says Democrats Have No Counter Offer, McConnell Calls On Biden For Help

Addressing the Senate, Republican Minority Leader Mitch McConnell said that GOP Senate leaders submitted an offer to Democrats last night at 7:10 p.m., but have yet to receive a counter-offer. Now he is calling on Vice President Joe Biden to step in and help.

"We're willing to work with whoever can help," McConnell said. "I'm willing to get this done, but I need a dance partner."

Senate Majority Leader Harry Reid then responded, saying that Democrats haven't been able to come up with a counter-offer. The issue is apparently McConnell's demand for the chained CPI.

“The Republican leader has told me and he’s just said that he’s working with the vice president,” Reid said. “And he and the Vice President, I wish them well. In the meantime, I will try to come up with something, but at this stage I don’t have a counter-offer to make. Perhaps as the day wears on, I will be able to. But I think the Republican leader has shown absolutely good faith.”

REID: 'We're Not Going To Have Any Social Security Cuts' In Fiscal Cliff Deal

Senate Majority Leader Harry Reid returned to the Senate floor to clarify that the major issue blocking a deal is the Republicans demand for the chained CPI.

"We're not going to have Social Security cuts as part of a smaller, short-term agreement," Reid said. "At some point in the negotiating process becomes obvious one side is demanding concessions they know other side's unwilling to make."

But he added that the two sides remain "real close" to a deal.

"I am not overly optimistic, but I am cautiously optimistic that we can get something done," he concluded, adding that he hopes that he and McConnell will put out a joint statement this afternoon.

Lindsey Graham: 'I think we're going over the cliff"

After expressing confidence Sunday morning that the Senate could reach a deal to avoid the cliff, South Carolina Republican Sen. Lindsey Graham apparently no longer believes that a compromise can be reached.